FIG. 1 shows an optical modulator that is based upon the principle of the Mach-Zehnder interferometer, commonly referred to as a MZI modulator. The modulator comprises an optical waveguide receiving a power P, which is divided into two branches at a point S. The two branches join again at a point J, one directly and the other via an electro-optical phase shifter 10. Each branch carries half of the original optical power. An optical wave may be phase-shifted, because it acts as a carrier of frequency f=c/λ, where c is the speed of light, and A the wavelength. The carriers arriving via the two branches are added at point J of the modulator, one having been shifted by φ by phase shifter 10. The resulting carrier has a power of P·cos 2(φ/2), neglecting the optical losses.
FIG. 2 shows a perspective view of a phase shifter 10 inserted between two aligned sections of an optical waveguide 12. As shown, the waveguide has an inverted “T” cross-section, the central part of which carries the optical beam. The phase shifter 10 is configured to connect to the sections of the waveguide, and it also has an inverted “T” cross-section. In addition, the edges of the phase shifter ascend above the plane of the waveguide; these edges bear electrical contacts (not shown) for controlling the phase shifter.
FIG. 3 is a so-called High-Speed Phase Modulator (HSPM) phase shifter 10. In this cross-section view, the section plane is perpendicular to the axis of the optical waveguide. A dashed line circle, at the thicker portion of the central zone, represents the portion of the waveguide crossed by the optical beam. The phase shifter comprises a semiconductor structure, typically silicon, forming a P—N junction 14 in a plane parallel to the axis of the waveguide, and offset relative thereto. The junction 14 is shown, for example, at the right side face of the waveguide. A P-doped region extends to the left of junction 14, and has a cross section conforming to the cross section of the waveguide, namely elevated in the center and lower at the edge. Zone P ends at its left by a P+ doped raised region, bearing an anode contact A.
An N-doped region extends to the right of junction 14 and conforms to the cross-section of the waveguide. Zone N ends at its right by an N+ doped raised area, bearing a cathode contact C. The structure of the phase shifter may be formed on an insulating substrate, for example, a buried oxide BOX.
For controlling the phase shifter of FIG. 3, a voltage is applied between the anode and cathode contacts A, C, which reverse biases the junction 14 (the ‘+’ on the cathode and the ‘−’ on the anode). This configuration causes a displacement of electrons e from the N region to the cathode and of holes h from the P region to the anode, and the creation of a depletion region D in the vicinity of the junction 14. The carrier concentration is thus modified in accordance with the magnitude of the bias voltage, in the area crossed by the optical beam, which results in a corresponding modification of the refractive index of this area. The sensitivity of an electro-optical phase shifter is expressed in degrees per volt per unit of length of the phase shifter. The sensitivity of the phase shifter to changes in the control voltage increases with its length, but the absorption coefficient, i.e. the optical losses, also increases with the phase shifter's length.